In the development of screen cylinders and plates having a screening surface and an accepts surface on opposite faces thereof, for screening pulp flowing in a flow direction, to separate accepts from rejects, the introduction of contoured cylinders and plates was a dramatic breakthrough in the art. Particularly with the contoured cylinders of U.S. Pat. No. 4,529,520 (the disclosure of which is hereby incorporated by reference), sold commercially under the trademark PROFILE.RTM. by CAE ScreenPlates Inc. of Glens Falls, N.Y., it has been possible to increase debris removal efficiency dramatically while maintaining throughput, or to increase throughput dramatically while maintaining the same debris removal efficiency. The particular surface manifestations defining the contour of such cylinders direct flow toward the slots or round holes in the screen surface (connected to the accepts surface) for maximum throughput, while causing the flow of pulp to turn, inducing micro-turbulence which fluidizes the pulp at the slots or holes, facilitating passage of accepts through the slots or holes. Such cylinders thus have optimized debris removal efficiency since they can run smaller holes or slots than in any earlier screen design without negatively impacting throughput.
While contoured screens have become the standard in the industry now, and are used in more than half of the pressure screen machines in North America, such screens have not been as enormous an advance in the art with respect to the screen's ability to operate at smaller reject rates, as compared to the increase in debris removal efficiency provided thereby. An operational parameter that is significant is the screen's ability to concentrate the rejects (that is the reject, or undesirable, material, such as shives) in the rejects stream and thereby reject out less of the desirable material into the rejects stream (providing it in the accepts stream instead). This may in part be due to the inclination of the reject material (e.g. shives) to form a mat or layer at the entry side of the screen plate or cylinder, hindering the passage of accepts through the screen slots or holes.
As taught by co-pending application Ser. No. 08/286,440 filed Aug. 4, 1994 [atty. dkt. 1497-21], the disclosure of which is hereby incorporated by reference herein, the selection of contour screen slot or hole configurations is important, including a requirement for more aggressive contours at the reject end. The present invention takes this contour selection procedure a step further and introduces a unique macro/micro-turbulence creating environment suitable in some applications only at the reject end of the cylinder, while in others over the entire cylinder.
According to the present invention, a screen cylinder or plate construction is provided which retains all of the benefits of the conventional contoured screen cylinders, such as the PROFILE.RTM. screen cylinder, while also improving the screen cylinder's ability to operate at smaller reject rates. According to one aspect of the present invention, this is accomplished by providing a particular contour of the screen surface (particular surface manifestations) so that the reject material is maintained highly turbulent. According to the present invention shear stresses are induced into the reject material so that the reject material is kept in substantially constant motion.
According to one aspect of the present invention a screen so cylinder having a screening surface and an accepts surface disposed on opposite faces thereof, for screening pulp flowing in a flow direction to produce accepts, and rejects comprising reject material, with screen openings extending between the screening and accepts surfaces, is provided. The screen cylinder includes: A plurality of substantially parallel grooves disposed substantially perpendicular to the flow direction, formed in the screen surface, and including surface manifestations for inducing shear stresses in the reject material flowing with pulp in the flow direction to keep the reject material in substantially constant motion to prevent a mat or layer of reject material forming on the screening surface, and for inducing micro turbulence into pulp flowing in the flow direction at the immediate area of the screening openings at the screening surface, to fluidize the pulp thereat so that accepts readily pass through the openings toward the accepts surface.
According to another aspect of the present invention, a screen cylinder or plate having a screening surface and an accepts surface on opposite faces thereof is provided. The screen cylinder or plate comprises the following elements: A plurality of substantially parallel macro grooves formed in the screening surface generally transverse to the flow direction. A plurality of substantially parallel micro grooves formed in at least a portion of the screening surface within each macro groove and generally transverse to the flow direction. And, each of the micro grooves comprising contoured surface manifestations which induce micro-turbulence to fluidize pulp at the immediate area of screening, and at least one opening extending between the screening surface and the accepts surface for allowing passage of accepts therethrough.
In the screen cylinder or plate described above, preferably each of the micro grooves has substantially the same contour of surface manifestations, and the macro grooves each have a contoured configuration that simulates the contour of each of the micro grooves within it. Typically about four micro grooves are provided in each of the macro grooves (normally between three and five), and they are formed in the entire screening surface, or only that portion adjacent (e.g. the closest one-tenth to one-half of the cylinder) the rejects end of the cylinder.
The micro grooves may have any desired contour and surface manifestations which induce micro-turbulence in the pulp. Of course the configuration of the PROFILE.RTM. commercial cylinder and equivalents thereto are most desirable (equivalents being shown in U.S. Pat. Nos. 4,529,520, 4,950,402, 5,000,842, 4,880,540, and 5,073,254, among others, and in PCT published application WO 91/05911 published May 2, 1991, and Japanese published patent application no. Hei 2-264092 published Oct. 26, 1990), however other configurations can also be employed.
The desired configuration for the screening surface, which is similar to the PROFILE.RTM. conventional screen cylinders, includes an upstream wall generally transverse to the flow direction, a depression located closer to the accepts than the upstream wall, and a downstream wall extending gradually from the depression toward the upstream wall of the next groove, the at least one opening (slot or drilled hole) extending at least in part from the depression to the accepts surface. While the upstream wall can be completely perpendicular (that is make a 90.degree. angle with respect to the flow direction), most desirably it makes an angle of about 70.degree.-130.degree. (preferably about 95.degree.-105.degree.), and is substantially planar. The downstream wall also is preferably substantially planar and makes an angle of about 5.degree.-60.degree. with respect to the flow direction. The depression may be defined by a continuation of the downstream wall, or may be a substantially flat surface substantially parallel to the flow direction, and the slots or holes are preferably provided in that substantially flat surface, or whatever surface is adjacent the upstream wall. The slots or holes are preferably located as close to the upstream side wall as practical.
The configuration also preferably comprises first substantially flat portions substantially parallel to the flow direction provided between the downstream side wall of one micro groove and the upstream side wall of the next, with second substantially flat portions substantially parallel to the flow direction being provided between adjacent macro grooves. The macro grooves preferably each have an upstream side wall that is not part of a micro groove. While the upstream side wall of a macro'groove is preferably also generally transverse to the flow direction, it typically makes an angle of about 100.degree.-130.degree. (preferably about 100.degree.-110.degree.) with respect to the flow direction, preferably an angle greater than the angle of a micro groove upstream wall. Also the length, in the flow direction, of a second flat portion is preferably greater than the length of a first flat portions.
Each of the downstream walls of the micro grooves also has a peak portion, which is the portion of the downstream wall most remote from the accepts surface. These peak portions preferably are provided in substantially a straight (imaginary) line, which line makes an angle of about 5.degree.-15.degree. (e.g. about 8.36.degree.) to the flow direction, the line extending from the first micro groove within a macro groove in the flow direction, to the last micro groove within a macro groove in the flow direction.
According to another aspect of the present invention a screen cylinder or plate having a screening surface and an accepts surface is provided which comprises the following elements: A plurality of substantially parallel consecutive grooves formed in the screening surface, each groove having at least one opening extending between the screening surface and the accepts surface, at an opening-receiving area spaced from the accepts surface; and at least three substantially consecutive grooves have significantly different spacings of the opening-receiving areas thereof from the accepts surface. The at least three substantially consecutive grooves preferably comprise four or five grooves per pattern, and are disposed in sequence so that in the direction of flow the spacings of the opening-receiving areas thereof from the accepts surface increase. The consecutive grooves with different spacings are preferably in repeating patterns substantially completely covering the screen surface.
The invention also relates to a pressure screen for screening pulp utilizing a screen cylinder as described above. The pressure screen also includes a foil, means for moving the foil and the screen cylinder with respect to each other (typically one or both are rotated), an inlet for pulp, an accepts outlet, and a rejects outlet.
According to still another aspect of the present invention a method of screening pulp having a consistency of about 1-5% to produce accepts and rejects is provided. The method comprises the steps of simultaneously: (a) Moving the pulp in a flow direction so that it moves relative to the screening surface. (b) Inducing shear stresses in rejects material adjacent the screening surface to keep the rejects material in constant motion so that the rejects material does not form a mat or layer on the screening surface. And, (c) inducing micro turbulence in pulp immediately adjacent the screening openings to fluidize the pulp thereat so that accepts readily pass through the openings toward the accepts surface. Steps (a) through (c) may be practiced at a reject rate of about 3-10% (e.g. about 5%), while having a debris removal efficiency (Ec) of above about 85%. The screen has a rejects end, and steps (b) and (c) may be collectively practiced only adjacent (e.g. in the bottom one-tenth to one-half) of the rejects end.
It is the primary object of the present invention to provide screen plates or cylinders operable at small reject rates to efficiently screen rejects from pulp, the screen cylinders or plates utilized with pressure screens in an advantageous method of screening pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.